State of the art “solar tower” type solar-thermal energy systems (power plants) use large numbers (e.g., one thousand, or often more) of heliostats to reflect sunlight onto a tower-mounted (raised) solar receiver for conversion to usable electricity. Each heliostat typically includes an array of flat (or in slightly concave) mirrors that are maintained in a substantially upright position on a support post. A total reflective surface area per heliostat of greater than 100 m2 is not uncommon, yet there is a trend observable in recent plants towards smaller, less wind-affected, heliostats with individual mirror areas as small as 1 m2. Each mirror in the array (heliostat field) is pivoted (rotated) in two axes to track the apparent angular movement of the sun such that exiting (reflected) sunlight is constantly directed from the mirrors onto the raised solar receiver during daylight hours. A prominent example of a conventional solar tower system is the PS20 plant near Seville, Spain, which is built by Abengoa Solar from the same sunny European country. PS20 produces 20 MW of electricity from collecting sunlight from 1,255 heliostats, with each heliostat having a flat mirror surface area of 1,291 square feet. Across the Atlantic, heliostat development effort in the U.S. was initiated in 1975. Since then, solar tower plant (system) designers determined that it would be more economical to build larger heliostats which in turn will service plants with larger power output. These plants are very promising as a renewable power source because the LCoE (Levelized Cost of Energy) is near 6 to 7¢/kWhr, which falls somewhere between the U.S. retail rates of 10/kWhr and generation cost from fossil fuel plants of 3¢/kWhr. Cost subtotal of heliostats makes up 50% of the total cost of a solar tower plant, and current technology has not observably brought the cost of heliostats down below 100$/m2, based on mirror surface area.
The solar tower industry has to overcome a number of technical challenges to bring future cost of heliostats to below $100/m2, at which point experts believe that the solar-tower technology will be competitive on the open market, especially if carbon-offset trading becomes the norm.
One impediment to reducing the cost of conventional heliostats is that the upright mirror arrangement experiences significant wind loading that must be accounted for by the mirror frame and support post. In windy conditions, the upright mirror arrangement effectively forms a large wind sail, and the resulting wind load forces are transmitted through the mirror support frame to the support post (which acts as a mast). Unless the support frame and support post structures are engineered to withstand worst case wind conditions, they risk damage or complete failure (collapse) under worst-case wind conditions. Thus, each heliostat's support frame and support post structures must either be extensively engineered, resulting in high design and production costs, or the heliostats will be subject to periodic wind-related damage, resulting in high repair and/or replacement costs.
Another problem facing conventional heliostats is that the upright mirror arrangement necessarily requires maintaining motors and/or complicated linkages at a significant distance above the ground in order to effect the necessary two-axis sun tracking operation. Periodic maintenance of the elevated location of the motors/linkages requires expensive lift equipment to enable access to the mirror array, and requires the maintenance personnel to work high above the ground and to move between the spaced-apart operational areas of the mirror array, thus increasing maintenance cost and chance of injury.
Another problem facing conventional upright mirror heliostats is that the heliostats must be positioned at a conservative offset spacing in order for all mirrors of conventional upright mirror arrangements to receive/reflect sunlight at most times during the year (i.e., in order to avoid shading/blocking of the mirrors). Depending on latitude and exact solar farm layout, a yearly aggregate of between approximately 30 and 80 percent more sunlight is available within the standard footprint of a standard upright mirror solar tower plant than is actually reflected by the heliostat mirrors. This “extra” (unreflected) sunlight is directed onto the bare ground between the heliostats that results from the conservative offset spacing, and the wasted “extra” sunlight is at a maximum when the noontime sun elevation angle is near its zenith (e.g., within one month of the summer solstice). Therefore, the ground coverage ratio (i.e., the ratio of reflected/captured sunlight to the total sunlight directed onto a given solar farm footprint) associated with conventional upright-mirror heliostats is necessarily small in a year-round aggregate, thus requiring a relatively large amount of land to produce a desired amount of solar power.
What is needed is an improved heliostat that addresses the cost, maintenance and ground coverage issues associated with conventional heliostats. What is also needed is a solar tower system that utilizes the improved heliostat in an efficient manner.